Growing evidence supports the potential for creatine, a critical regulator of energy homeostasis in muscle and brain tissue, to function in an antidepressant manner. Studies indicate that creatine supplementation can significantly improve mood in treatment resistant depression, and it is hypothesized that creatine alters brain chemistry in a manner that increases the likelihood of response to antidepressant treatment. Over the last two years, our lab has been actively studying the effects of chronic creatine supplementation on depression-like behavior in rats using the forced swim test, an animal model that is selectively sensitive to agents that alter depressive symptoms in humans. Importantly, these studies show that long-term supplementation with creatine results in reliable sex-specific alterations in depression-like behavior (Allen et al, 2009). Specifically, creatine produces antidepressant-like effects in females and pro-depressant effects in males. These findings are supportive of the report by Renshaw et al (2001) that altered purine levels in depressed women, but not men, are associated with treatment response. Insight into the mechanisms underlying these effects is of great clinical importance as sex differences in the prevalence and treatment of depression in humans are widely reported. Preliminary research suggests the antidepressant-like effect of creatine in females may be explained by the phosphorylating action of estrogens or estrogenic metabolites upon creatine kinase, an enzyme that stimulates the consumption of brain energy stores. The primary aim of the present research is to determine whether the sex-specific effects of creatine supplementation on depression-like behavior are attributable to the actions of gonadal steroids at the time of forced swim testing using classic hormone manipulation paradigms. That is, this work will assess whether the effects of creatine can endure in the absence of ovarian or testicular hormones. A secondary aim is to examine whether there is a relationship between creatine and neurotrophic- related neuronal activity. Evidence points to a number of shared qualities among creatine and brain-derived neurotrophic factor (BDNF), an essential mediator of synaptic plasticity that is robustly linked with depression. BDNF diversely interacts with gonadal steroids and may be one molecular mechanism involved in the creatine- induced alterations in depressive behavior. If sex-dependent behavioral phenotypes persist after gonadal steroid manipulation, studies are planned to assess the organizational effects of sex hormones on depressive responsivity in male and female rats supplemented with creatine. It is hypothesized that estrogens mediate the antidepressant effects of creatine, and that creatine and BDNF are differentially expressed in male and female rats. The examination of hormonal and neurotrophic variables with creatine and behavior is novel, and the biochemical data from these studies will be valuable in elucidating the underlying mechanism of creatine's behavioral effects. Together these studies will fill critical gaps in knowledge regarding the sexually dimorphic neural and behavioral consequences of creatine supplementation on depressive behavior. PUBLIC HEALTH RELEVANCE: Given the mounting functional and clinical significance of creatine in the brain, it is essential to more fully characterize the role of this agent in the pathophysiology of mood disorders. Moreover, examining mechanisms underlying sex differences in depression-like behavior in rats in response to creatine treatment will profoundly affect our understanding of creatine, its relationship with depressive behavior, and may lead to sex-specific therapeutic strategies. This research is highly relevant to public health interests, as creatine is among the most extensively used nutritional supplements to date and the potential for creatine to alter mood may directly impact management of depression.